GPS requires an unobstructed line of sight to a number of satellites. Therefore, in locations with restricted visibility of the open sky, GPS is normally unavailable. One example of such an environment, though not the only field of application for this invention, is an indoor setting. Another example is outdoors in an urban area or a wooded area where there is limited visibility of the sky.
The problem of indoor location detection has been addressed in different ways. Custom Ultra Wide Band (UWB) hardware units can be installed at regular intervals throughout a facility, to spread signals throughout the facility, and custom hardware tags can be attached to assets that are to be tracked. The approach is accurate and has been adopted by manufacturers such as car and airplane makers to track the location of components and vehicles, but is expensive and only affordable for high end manufacturing processes.
Any system relying on tracking physical tags is not applicable to allowing end users to employ standard consumer technology such as mobile phones and PDAs to determine their location in an indoor environment or other environment where GPS is unavailable or unreliable.
Another approach is found in U.S. Pat. No. 7,876,266 which replicates a GPS-like solution using wireless signals. It requires several collaborating devices to generate solutions which satisfy the accumulation of time-stamped pings received by the plurality of devices from multiple nodes.
WO 2011/014292 discloses a system based on ultrasonic detection. Transmitters in a location such as a store transmit identifiers encoded in an ultrasound signal. The signal is received by a microphone of a mobile phone, allowing an app on the phone to infer location. Granularity is limited to determining that the phone is within range of a given transmitter, e.g. that the device is within a store.
US 2005/0131635 discloses using a probabilistic model of a wireless environment such as a wireless local area network or WLAN. The model indicates a probability distribution for signal values at several sample points in the wireless environment. A set of observations of signal values is made and the target device's location is estimated based on the probabilistic model and the set of observations. An error estimate is determined as a combination of products over several sample points, with each product comprising a probability distribution for the sample point in question being the target device's location, and a distance function between the sample point in question and the target device's estimated location.
Approaches based on signal strength, such as US 2005/0131635, require a probabilistic model of some sort. Signal strength-based location detection can be carried out in a region provided with a number of routers or wireless access points (WAPs) which together provide a wireless local area network (WLAN) or a number of such networks. Regardless of whether the WAPs are part of the same network or are in different networks, a user's mobile device can detect each WAP by its MAC address or basic service set identification (BSSID) which the WAP broadcasts at regular periodic intervals. Thus, a wireless device can scan the relevant portion of the spectrum, identify broadcast packets, and identify each WAP within range. For each WAP the signal strength can be (and typically is) measured as a decibel value.
In theory, therefore, at any given point within the region there should be a unique wireless signal environment or fingerprint composed of signals each of a different strength. The fingerprint should be unique because signal strength is primarily governed by an inverse square law, and with multiple WAPs surrounding a device each point in space should be uniquely identifiable as a set of signal strengths each of which follows this law.
In practice however the situation is not as clean as theory would suggest. Signal strengths measured by a device fluctuate and are not constant, even if the WAP and device are stationary. The inverse square law does not take account of a physical environment such as is typically found indoors or in areas with walls, partitions, or obstacles, where any signal can exhibit the effects of blockage, attenuation, reflection, interference (both with itself and with other signals, both from the wireless network and from other sources). It is therefore observed that attempts to locate a device by matching the detected set of observed signal strengths to a database of previously observed signal strengths at the same location, provides poor results which may be ambiguous or which may even give an entirely wrong location. This problem is exacerbated in some environments more than others, and is worse with some devices than with others.
Other approaches to location detection known to the inventor are described in U.S. Pat. No. 7,522,049; US 2011/0018769; U.S. Pat. No. 8,150,367; WO 2012/170389; U.S. Pat. No. 7,933,612; EP 2 217 942; EP 2 307 903; US 2012/0258669; US 2012/0225663; US 2011/0029370; US 2011/0029359, and US 2011/0028160.